The present invention relates to a data processor provided with a CPU and formed on a single semiconductor substrate, and to the technique of identifying a tone wave in an analog signal including multiple frequencies dealt with by such a data processor, and particularly to the technique applied effectively to a single-chip microcomputer LSI having DTMF (dual tone multifrequency) receiver function.
The DTMF signal used by the push-button telephone is a 2-frequency signal having, in combination, one frequency out of four low-band frequencies and one frequency out of four high-band frequencies. The DTMF signal has been used for specifying a destination station at calling, and is now used more extensively for inputting a password for reviewing automatic telephone messages and for signals of telephone-based home automation facilities and remote control facilities.
Conventionally, receivers for receiving a DTMF signal and separating its tone wave have been solely available in tie form of special LSI devices. For example, a DTMF receiver LSI device separates a tone wave from the DTMF signal using a high-band band-pass filter and low-band band-pass filter and shapes the waveform of the separated tone wave using a comparator or limiter. The device measures the interval or period of the shaped signal on the basis of a clock signal and compares the result with preset expected values to identify the tone included in the 2-frequency signal, or otherwise identifies the tone included in the 2-frequency signal through the provision of eight-channel switched capacitor filters for the discrimination among four high-band frequencies and four low-band frequencies. The result this identification is decoded by a decoder or the like, and fed out of the DTMF receiver, and delivered to a data processing LSI such as a processor or microcomputer by which an intended control is carried out. Such an LSI DTMF receiver is described, for example, in "LSI Handbook," p. 629, published by Ohm on Nov. 30, 1984.
However, if the DTMF receiver is formed of an LSI device different from the associated data processor which performs control on the basis of the receiver output, it can be susceptible to noises resultant from the interface therebetween. Furthermore, the conventional DTMF receiver LSI is designed to output externally the identification result of tone wave included in the multifrequency signal, and therefore it merely identifies limited tones, or in other words it is merely applicable to input analog signals having limited frequency bands. The DTMF receiver has its characteristics and function determined by its inherent hardware arrangement and therefore it lacks in flexibility in the tone frequency to be discriminated and in the frequency band of the input analog signal. Moreover, it has a fixed sampling rate for conversion and a fixed conversion accuracy, and therefore does not allow the selection of an operating mode and function, which is required for adapting the device to various systems and specifications in demand. These are the prior art deficiencies revealed by the inventors of the present invention.